Processes for separating small particles contained in a suspension or slurry by size find application in various industries. The ability to make such separations is particularly desirable in paper making since the thickness and length of the pulp fibers are strongly related to the quality and characteristics of the paper produced from the fibers. Several specific potential uses in the paper industry for efficient fractionation processes have been identified. A pulp slurry formed of reclaimed waste paper or paperboard could be fractionated to remove clumps and particular contaminants, and to separate fibers above and below a desired size. For example, such fractionation would allow the "linerboard" fibers in a slurry of waste corrugated fiberboard to be separated from the "medium" fibers. Linerboard is mainly composed of softwood fibers of relatively large size (40-50 microns diameter, 3-5 mm length) whereas medium fibers are mainly hardwood fibers of smaller size (20-30 microns diameter, 1-3 mm length).
Fractionation would also allow a single fiber source, which ordinarily is a mix of fibers of various sizes, to be used optimally in the production of a desired multilayered product. Each fraction, separated by fiber size, could be used to form a single layer which would have characteristics reflecting the size of the fibers in the layer. The layers of different fractions would then be combined to form a multilayered product with qualities not possessed by a single layer product formed from the original fiber mix.
The separated pulp fractions could also be used alone to make single layer products having desired characteristics related to fiber size. In addition, some papermaking machines operate most efficiently with pulp having a particular fiber size range. Another potential application of pulp fractionation is the separation of a pulp stream into two or more fractions which can be beaten separately under optimum conditions and then recombined.
Although the potential applications of pulp fractionation are well known, fractionation has not been commercially important because of the lack of efficient equipment. Commercial equipment capable of pulp fractionation, e.g., centrifugal screens and the Johnson Fractionator, generally suffer from high energy consumption, a requirement for low pulp concentration (1% or less), and potential water pollution problems if operated on a large scale.
In addition to the commercially available fractionation processes, it has been found that if a pulp slurry is fed to the underside of a conventional, commercially available, rotating disk atomizer of inverted saucer geometry, the resulting spray will show a variation, as a function of vertical position, in the average size of the granular and fibrous particles in the spray. See, e.g., K. Moller, et al., "Screening, Cleaning and Fractionation with an Atomizer," Paper Technology and Industry, Vol. 20, No. 3, pp. 110-114, April 1979, which also proposes two physical mechanisms to explain the fractionation phenomenon. First, a high shear gradient is assumed to exist in the pulp film on the atomizer wheel. The portion of the pulp suspension near the wheel surface would be accelerated more quickly than the portion of the suspension near the free surface of the film. The high shear gradients near the wheel surface cause the larger particles to migrate away from the immediate vicinity of the wheel surface while the fine material stays behind Another mechanism proposed for the fractionation is based on the centrifugal force experienced by the film as it moves over the surface of the inverted, saucer shaped atomizer disk. This force serves to keep the film, as a whole, pressed hard against the atomizer surface and thereby maximizes acceleration. The centrifugal force is presumed to cause the larger, denser particles or fibers to migrate inwards from the free surface of the film toward the surface of the wheel with the smaller particles or fines remaining behind.
Fractionation tests in which the particle slurry is fed to the underside of an atomizer wheel show that the concentration of smaller particles in the spray surrounding the wheel increases gradually with increasing height while the concentration of larger particles decreases. Thus, by collecting a portion of the spray at a selected position in the spray, it is possible to obtain a particle mix which has a higher percentage of particles of a certain size than does the feed stock. However, because of the apparent gradual change in particle size as a function of verticle position around the atomizer wheel, commercial atomizer equipment does not provide efficient fractionation and generally cannot be used to obtain a fractionated product which contains only particles within a specified size range or which is free of particles in a specified size range.